In commonly owned U.S. Pat. No. 3,792,316 there has been described such a system in which the output current is fed to an external connector by way of a calibrated wire. The system includes monitoring means for measuring the current delivered by the power element directly to the calibrated wire.
In some practical applications of the device described in the above-identified patent it has been found that the resistance of the calibrated wire extending to the external connector, even if only of the order of a few tens of milliohms, is too great and, therefore, in the case of high currents, the wire heats up excessively, causing an undesired increase in the temperature of the integrated circuit and, accordingly, a limiting of the power output of that circuit.
Furthermore, in the system described in the aforementioned patent, the protective circuit for a power transistor is designed in such a manner that the resultant relationship between the current which passes through the power transistor and the voltage present between the terminals thereof is of exponential character, on account of the presence of a diode in the circuit for the processing of the variables which are being monitored. By suitably selecting the value of the components of the circuit, this relationship is represented, in the characteristic diagram of the power transistor by a curve which approaches partly the curve of maximum dissipation and partly the second breakdown curve of the power transistor, as shown in the graph given in FIG. 3 of U.S. Pat. No. 3,792,316. In this way, the transistor is enabled to operate safely under conditions very close to those limits, thus optimally utilizing the integrated circuit.
It has been noted, however, that with many types of power transistors the variations of the parameters permitted by the aforedescribed circuit are not sufficient to obtain satisfactory approximation of the second breakdown curve. In many cases, therefore, the protective circuit excessively limits the useful range of operation of the transistor, namely the power which can be dissipated by it.